THE CAUSE OF THE GLACIAL PERIOD. 503 



were caused : first, by abstraction of water from the ocean and 

 its deposition on the land as snow, which under pressure made 

 the vast ice-sheets ; and, second, by ice attraction of the ocean, 

 lowering it still further, except in the vicinity of glaciated 

 lands. An area of about 4,000,000 square miles in North 

 America, and another of about 2,000,000 square miles in 

 Europe, were covered by ice-sheets, which in their maximum 

 extent had probably an average thickness of a half or two thirds 

 of a mile, or perhaps even of one mile. Assuming that these 

 ice-sheets were contemporaneous, but disregarding ice-fields of 

 smaller extent, which probably existed in parts of Asia and of 

 the southern hemisphere, as also the glaciers of mountain dis- 

 tricts, the lowering of the ocean surface, which covers approxi- 

 mately 145,000,000 square miles, would slightly exceed 100 

 feet, if the mean depth of the ice accumulation was half a mile. 

 More probably the sea over the whole globe was thus depressed 

 fully 150 feet, which would correspond to an average of about 

 3,600 feet of ice on the glaciated areas of North America and 

 Europe. For the second factor in causing such changes, Mr. 

 R. S. Woodward's computations* indicate that gravitation 

 toward the ice would further depress the ocean probably 

 twenty-five to seventy-five feet within the tropics and in the 

 southern hemisphere, while it would raise the level enough 

 near the borders of the ice-sheets to counterbalance approxi- 

 mately the depression due to the diminution of the ocean's 

 volume, and would lift portions of the North Atlantic and of 

 the Arctic Sea perhaps two or three hundred feet higher than 

 now. Stream erosion while the sea was lowered to supply the 

 ice of the Glacial period may explain the indentations of the 

 southeastern coast of the United States, as Pamlico and Albe- 

 marle Sounds, besides similar inlets in many other parts of the 

 world ; but the excavation of Chesapeake and Delaware Bays 

 seems more probably referable, at least in part, to the time of 

 preglacial elevation, with the channeling of the now sub- 

 merged Hudson fiord. 



When the ice-sheet of the last Glacial epoch finally re- 



• " United States Geological Survey, Sixth Annual Report/' pp. 291-300 

 and " Bulletin No. 48," " On the Form and Position of the Sea-Level." 



